A barge eases southbound in a Mississippi River channel toward a lock that remained shut down Wednesday, Sept. 19, 2012, in Granite City, Ill., causing a traffic jam of doz-ens of tugboats and hundreds of barges. The river at that stretch was shut down since workers last weekend discovered damage to a vertical, rock-filled steel tube called a "protec-tor cell" that serves as a buffer that barges rub against, aligning them before they actually enter the lock. (AP Photo/Jim Suhr)

U.S. Army Corps of Engineers Col. Chris Hall points to a vertical, rock-filled steel tube called a "protector cell" that serves as a buffer that Mississippi River barges rub against, aligning them before they actually enter the lock on Wednesday, Sept. 19, 2012, in Granite City, Ill. Emergency repairs are under way on that structure after workers discovered that an unarmored portion of it exposed by river levels dramatically lowered by the drought split open, spilling much of the rock into the channel and forcing the channel to be closed to shipping. (AP Photo/Jim Suhr)

GRANITE CITY, Ill. (AP) — The busiest lock on the Mississippi River is back in business after days of emer-gency repairs caused a shutdown of that stretch of river near St. Louis, backing up hundreds of barges.

The U.S. Army Corps of Engineers reopened Lock 27 at Granite City at about 3:30 a.m. Thursday, five days after it was put out of action when crews noticed damage to what’s called a "protection cell."

That’s a rock-filled steel cylinder against which barges rub to help align them for proper entry into the lock. That buffer had split open, spilling enough of the rock into the river to obstruct passage.

By the time the lock finally was reopened, the Coast Guard says the traffic jam had grown to 63 vessels and 455 barges.

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